The Shells tell the Truth: Molluscs, some Stratigraphic Order and early Evolution

"He was painter, and he used his art to vividly depict his own concepts. So he depicts on the frontispiece the spirit of observation who, climbing on a mountain on which ground are spread marine bodies, shows one of those to a somehow surprised ghost, emerging from the mist, which seems not able to believe his own eyes."Brocchi (1814) describing the work of his predecessor, artist and naturalist Agostino Scilla. Scilla in 1670 published “La vana speculazione disingannata dal senso”, where he argued that fossils are the petrified remains of once living beeings and one simply had to observe the similarities between the fossils and recent marine organisms in the field, instead of vain philosophising, to recognize this simple truth.

Italian mineralogist Gian Battista Brocchi (1772-1826) may not well known nowadays, but as both Charles Lyell as Charles R. Darwin were influenced by his works, he significantly contributed to forge modern geology. He studied jurisprudence and theology in the Italian town of Padua, but soon became interested in geology, mineralogy, botany and zoology and will frequently combine or even merge his various interests. Brocchi, who studied also ancient Egyptian art, argued that certain cultural phases and art-styles first appeared in a rudimentary form, developed over time to become more elaborated and finally would become obsolete and replaced by other, more modern, art-styles. Strongly influenced by this observation he applied a similar approach to botany and zoology, where he compared recent species with fossil ones, noting that similar to cultural phases, also species can become extinct and are replaced over time by new ones.

Also in the mineral kingdom he observed an “evolution” of minerals over time, even more curious, he argued on a sort of “descend with modification”. In his "Trattato mineralogico e chimico sulle miniere del Dipartimento del Mella" (1807-1808) he explained the great varieties of minerals as derived from a set of more simple rocks - as he states - “evolved” slowly over time according to natural laws acting now as in the past (it´s not a coincidence that such ideas sound similar to Lyell´s later uniformitarianism).

His observations and research as inspector of mines in the recently established (1805-1814) kingdom of Italy was published in 1814 in the two volumes of “Subapennine Fossil Conchology”, where he also resumes all his geological as paleontological speculations.

Volume one is an introduction, discussing stratigraphic as geological problems, volume 2 deals with the classification, description and distribution of Italian fossils found in Tertiary strata. Brocchi showed that different strata can be distinguished by the different species-assemblages, where the found fossil species become more and more similar to recent species as younger is the studied strata. Scottish geologist Charles Lyell, who was fluent in Italian and surely know of Brocchi´s work, will use the classification and distribution of fossil molluscs to define the geological epochs of the /former) Tertiary period. For Brocchi only the extinction of species and birth of new ones could explain the stratigraphic order he had observed.

Fig.3. Fossil shells (bottom, Pliocene-Pleistocene) showing already some striking similarities to recent (top) shells of marine snails and bivavlves found along the Adriatic coast. Brocchi and later Lyell observed that with decreasing age of the geological strata more and more "recent" molluscs appeared in the sediments.Unlike other contemporary naturalists like Comte de Buffon (1707-1788) and Georges Cuvier (1769-1832), Brocchi didn´t consider an external force (or catastrophes) necessary or responsible for the disappearance of a species from the geological record. Like a single human life or an entire culture, also a species would appear, evolve and thrive, but in the end vanish and disappear – it was inevitable and only a matter of time. Mammal species, appearing and disappearing quickly in the geological record, had a short “species-life”-expectancy. Molluscs, known for their longevity and also based on the observed stratigraphic range, had a longer life-expectancy and therefore slower faunal turnover. Brocchi surely know of Lamarck´s work on the fossil molluscs (completed in 1809) of the stratigraphic succession in the basin of Paris. Lamarck also subdivided geological strata based on fossil molluscs assemblages, but more important was an eager promoter of “variable species”. However from the contemporary reactions it seems that Lamarck´s s work was not well received. Lyell dedicated an entire volume of his "Principles of Geology" (1830) to rebut Lamarck´s hypothesis on transmuting species and Darwin considered the entire work of the French naturalist as "useless". Brocchi in contrast was at the time one of the few international recognized Italian naturalists and both Lyell´s uniformitarianism as the subdivision of the former Tertiary are based in part on Brocchi´s geological work. We know for sure that Darwin was strongly influenced by Lyell´s work and so surely came in contact with Brocchi´s geological ideas. Also some of Brocchi´s works on species evolution were translated into English and were probably discussed in lectures or in private meetings by the former teachers (like mineralogist Robert Jameson) of young Darwin.

Darwin during his voyage on the Beagle (1831-1836) wondered if species may die and are reborn in a discontinuous natural process.Observing fossils similar to bones of the modern Mara (Dolichotis patagonum), a South American rodent that resembles a small deer, Darwin realized that species were replaced in time by similar forms. However the young Darwin didn´t yet consider a gradual transition of one species into another possible, as he frequently refers to the observed animals as “individual species” and distinct entities in time. Only some time later, influenced by Lyell´s uniformitarianism, he will publish a more gradual model, where evolution doesn´t occur in jumps (as may suggested by Brocchi) but by slow and gradual evolution of populations. The distinct stratigraphic differences in species assemblages and the sharp limits between those, as used by Brocchi and Lyell to distinguish geological epochs, were for Darwin more an artifact caused by the"imperfection of the geological record" than by a supposed limited life-expectancy of a species. Species went extinct not for an organism-intern property, but simply because some species, by chance better suited to exploit the limited resources, would better survive and generate more offsprings and so over time replace less adapted species.